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In the 20th century, twin breakthroughs, quantum mechanics and general relativity, provided fresh insight into phenomena at the sub-atomic and cosmological scales, respectively. Yet physicists are still struggling to develop a consistent theory that bridges quantum mechanics and gravity. One approach to "quantum gravity" is string theory: a mathematical description of particles and forces at scales 1031 times smaller than a proton. String theory attempts to link particle physics, the forces of nature, and the earliest moments of the universe into a fresh theoretical framework. Hard evidence remains elusive, but theorists are applying string theory to important problems, such as explaining the exponential growth of the early universe and the paradoxes introduced to physics by the existence of black holes.